1. Field of the Invention
The present invention relates to an imaging apparatus, and, more particularly, to controlling the rotational velocity of a rotatable member, such as for example a feed roller, during initialization of an encoder used in an imaging apparatus.
2. Description of the Related Art
An imaging apparatus in the form of an ink jet printer forms an image on a print media sheet by ejecting ink from a plurality of ink jetting nozzles of an ink jet printhead to form a pattern of ink dots on the print media sheet. Such an ink jet printer may include a reciprocating printhead carrier that transports multiple ink jet printheads across the print media sheet along a bi-directional scanning path defining a print zone of the printer. Typically, a mid-frame provides media support at or near the print zone. The imaging apparatus includes a sheet feeding mechanism, which is used to incrementally advance the print media sheet in a sheet feed direction. One such sheet feed mechanism includes, for example, a feed roller driven by an analog DC motor, and having an encoder coupled to the feed roller for rotation therewith. In order for the encoder to function properly, however, the encoder must be initialized.
One example of an encoder initialization procedure is set forth in U.S. Pat. No. 6,452,512 B1, which is incorporated herein in its entirety by reference. In general, an analog encoder has a light element, such as an LED, and two photo sensors, such as photo diodes, defining the channel A and channel B output channels of the analog encoder. In the case of a rotary encoder, such as would be used with a rotating printer feed roller, a rotatable disk having a windowed mask may be positioned between the light element and photo sensors, which when rotated results in output signals being present on the A and B channels of the analog encoder. The analog encoder includes amplification and offset circuitry for each of the A and B channels. It is desirable that the channel A and channel B encoder signals be produced with the same amplitude and in the same range. However, variances between electrical components and mechanical imperfections tend to cause the signals produced by the photo sensors to differ in amplitude and range. Thus, such as encoder system has two primary parameters that can be adjusted in attempting to control the characteristics of signals present on the A and B output channels of the analog encoder; namely, the energization level of the light element and the direct current (DC) offset of the A and B channels from the photo sensors.
Adjusting light source energization tends to primarily vary the amplitude of the encoder signals produced, while adjusting the DC offsets of the photo sensors tends to primarily vary the range within which the encoder signals are produced. In addition to the different results obtained by adjusting encoder light element energization as compared to adjusting DC offsets, the timing involved in implementing such adjustments tends to differ greatly.
It has been determined that one challenge to be overcome in initializing the analog encoder is the difficulty in maintaining an appropriate feed roller velocity during analog encoder initialization. For example, if the rotational velocity of the feed roller is too slow, then the analog encoder wheel may stall during initialization, resulting in incorrect signal measurements and a failed calibration. As a further example, if the rotational velocity of the feed roller is too fast, then aliasing effects may occur, or imperfections of the encoder wheel, such as for example, ink spots, may not be detected and compensated for, resulting in unacceptable encoder signals.
What is needed in the art is a method and apparatus that facilitates closed loop encoder initialization using the encoder signals generated by the analog encoder, thus providing the desired rotational velocity of a rotatable member, such as for example a feed roller, that is needed during encoder initialization.